Characterization of hormones that regulate gonadotropin release has led to their application in clinical medicine and in production of foodproducing animals. Hypothalamic and gonadal hormones that affect LH and FSH secretion are used to enhance fertility, to treat infertility, for contraception and to synchronize reproductive cycles. Mechanisms regulating LH and FSH secretion are not fully understood. The apparent independent nature of plasma LH and FSH patterns have not been completely reconciled with observations showing that both hormones are produced in the same cell and that their secretion is jointly stimulated and inhibited by gonadotropin-releasing hormone and estradiol, respectively. Inhibin and activin may provide for independent regulation of FSH secretion. Evidence for inhibin as a gonadal hormone with negative feedback effects is ample. A role for activin as a circulating hormone promoting FSH secretion, however, is speculative. The goal of the project is to gain information that will strengthen or refute the putative physiological role of activin as a feedback hormone. The purpose is to determine whether the immunoreactive inhibin beta-subunit activity detected in systemic blood of swine reflects the presence of activin as well as inhibin. Swine are utilized because they produce substantial amounts of ovarian activin. Specific objectives are to: 1) mix 500 ml plasma pools with anti-alpha-subunit peptide immunoaffinity gel to clear molecules that possess alpha-subunit immunoreactivity, and then isolate any remaining molecules that have beta-subunit immunoreactivity, 2) determine whether any such alpha-subunit negative and beta-subunit positive immunoreactive molecules are FSH secretogogues when tested in vitro, 3) establish relative molecular weights of isolated material that expresses intrinsic FSH- releasing ability using an immunoblot technique, 4) assess the ovarian. contribution to plasma levels of activin-like molecules by comparing plasma from ovariectomized and intact pigs, and 5) process plasma collected at three stages of the estrous cycle to determine whether plasma concentrations of activin-like material are static or dynamic.